Capacitive position transducer

Abstract: A capacitive position transducer with reduced sensitivity to particulate and fluid contaminants. The electrodes of the position transducer are provided with a relatively thick coating of dielectric material so that the series impedance of the dielectric coatings is at least equal to the maximum capacitive impedance between the coatings whether or not there are any contaminants positioned therebetween. In one embodiment, the coatings are spaced-apart from each other to create an air gap while in another embodiment the coatings are resiliently biased toward each other so that the coatings slide along each other.

Description: This invention relates to capacitive transducers, and more particularly, to an apparatus for making such transducers less sensitive to liquid and particulate contaminants located between the capacitive electrodes of such transducers. Capacitive position transducers are becoming increasingly common in manufacturing industries. These capacitive transducers include a pair of substrates that move relative to each other along a measurement axis. A set of electrodes is carried by each of the substrates, with the electrodes of one substrate being positioned close to the electrodes on the other substrate to form a capacitor therebetween. The electrodes on each of the substrates are connected to conventional circuitry for providing an indication of the relative position between the two substrates as a function of the capacitance between various electrodes in each set. The capacitive position transducer may be of the incremental type, in which the circuitry provides only an indication of incremental movement from a known point, or an absolute position type, in which the circuitry provides an indication of the relative position between the two substrates regardless of whether their initial relative position is known. Capacitive position transducers of these types are disclosed in U.S.-A-4,420,754 and 4,879,508, Capacitive position transducers are often used as the principle component of electronic calipers for measuring thickness and other physical parameters. Although these calipers are sometimes used in dry, relatively, clean environments, such as inspection rooms or engineering offices, such calipers are often used to measure dimensions of work pieces in machine shops and other relatively dirty environments. When used in these environments, such calipers can become contaminated by particulate matter and fluids, such as cooling or cutting fluids. The liquid or particulate contaminants may find their way between each set of electrodes and affect the capacitance between the electrodes in a manner that is not related to the relative position between the substrates along the measurement axis. Contaminants between the electrodes of a capacitive position transducer degrade performance because the particulate or fluid may be a dielectric having a dielectric constant that is different from the dielectric constant of air that is in an air gap between the sets of electrodes. The capacitance between electrodes on opposite sides of the contaminate will thus be greater than the capacitance between other electrodes having the same relative geometry which do not have contaminants therebetween. Under these circumstances, the capacitance between the electrodes will not provide an accurate indication of the relative position between the substrates. One approach to minimizing the adverse effects of contaminants in capacitive position transducers is described in U.S.-A-5,172,485 which teaches coating the electrodes of each substrate with a thin layer of dielectric material and then mounting the substrate so that the dielectric material coating the electrodes of one substrate slides along the dielectric material coating the electrodes of the other substrate. In theory, the sliding contact between the dielectric layers eliminates any air gap for contaminants to fill. However, as a practical matter, the sliding contact approach described in US-A-5 172 485 cannot entirely eliminate an air gap. In fact, if there was no air gap at any area between the electrodes, very high frictional loads and wear would result. The absence of an air gap in any area between the electrodes also implies a perfect level of surface finish and flatness which is never achieved in practice. A practical best case design would still result in an air gap of 0.002 - 0.004 mm in some areas. For these reasons, the sliding contact approach described in the US-A-5 172 485 normally requires that the substrates be resiliently biased toward each other so that deviations from exact surface flatness and alignment can be accommodated by allowing the substrates to move apart. However, the compliant nature of the suspension for the substrates allows the substrates to be forced apart from each other by particulate contaminants which inevitably collect between the electrodes when the capacitive position transducer is used in a dirty environment. If the fluid contaminants were collected in the gap with a uniform thickness, the contaminants probably would not have an adverse affect on accuracy. However, the fluid contaminants are in fact collected between the electrodes in a highly non-uniform manner so that the thickness of the contaminant layer varies between zero and a substantial value. However, the particulate contaminants space the electrodes apart from each other by the maximum thickness of the contaminant layer. At areas between the electrodes where the fluid contaminant layer is thinner or not present, gaps are created which are filled with air. Thus, the sliding contact approach used to eliminate the air gap altogether has not proven to be adequate under most circumstances when used in contaminated environments. It is an object of the invention to provide a capacitive transducer that is relatively insensitive to particulate and fluid contaminants between the electrodes. It is another object of the invention to provide a technique for making capacitive transducers relatively insensitive to contaminants that can be used with a wide variety of capacitive transducer types. IEEE Transactions on Instrumentation and Measurement, December 1984 pp 276 to 280 discloses a capacitive transducer having a first substrate containing a first set of electrodes arranged along a measurement axis, and a second substrate containing a second set of electrodes arranged along the measurement axis, the first and second substrates being positioned with the first and second sets of electrodes opposite each other so that the capacitive coupling between the first and second sets of electrodes provides an indication of the relative position between the first and second substrates along the measurement axis, wherein a layer of dielectric material coats at least one set of first and second sets of electrodes thereby forming an air gap between an uncoated set of electrodes and the dielectric material when only one set of electrodes is coated, or between respective dielectric materials when both sets of electrodes are coated; According to the present invention such a transducer is characterised in that the ratio of thickness to dielectric constant of the dielectric when only one set of lectrodes is coated, or the sum of the ratios of thickness of dielectric constant for each dielectric material when both sets of electrodes are coated is greater than the thickness of the air gap, so that the maximum capacitive impedance between the first and second sets of electrodes in the air gap is less than the capacitive impedance between the first and second sets of electrodes in the dielectric material. In the accompanying drawings: Figure 1 is a cross sectional schematic view of a conventional, prior art capacitive position transducer having a substantial air gap. Figure 2 is a cross sectional schematic view of a capacitive position transducer in which the electrodes on each substrate are coated with a thin layer of dielectric and placed in sliding contact with each other. Figure 3 is a cross sectional schematic view of one embodiment of the inventive capacitive position transducer which is relatively insensitive to environmental contaminants. Figure 4 is a schematic representing the capacitances and impedances between the electrodes in the capacitive position transducer of Figure 3. Figure 5 is a cross sectional schematic view of another embodiment of the inventive capacitive position transducer which is relatively insensitive to environmental contaminants. A conventional capacitive position transducer 10 is illustrated in Figure 1. As is well known in the art, the transducer 10 includes an insulating substrate 12 that can be mounted on a structural member forming part of a caliper or machine tool, for example. A set of electrodes 14 are mounted on the substrate 12 in a predetermined pattern of conventional design. The substrate 12 may be a printed circuit board on which the electrodes 14 are formed by conventional means. The electrodes 14 are connected to a conventional electronics unit 16. An upper, insulating substrate 20 is mounted above the lower substrate 12 so that it can move relative to the substrate 12 along a measurement axis which extends from left to right as illustrated in Figure 1. A set of electrodes 22 of conventional design are mounted on the surface of the upper substrate 20 facing the electrodes 14 on the lower substrate 12. The upper substrate 20 may also be a printed circuit board on which the electrodes 22 are formed by conventional means. The electrodes 22, like the electrodes 14 on the bottom substrate 12, are connected to the electronics unit 16. The electrodes 14, 22 are separated from each other by an air gap 26 and thus form capacitors in which the electrodes 14, 22 are capacitively coupled to each other through the air dielectric in the gap 26. The electronics unit 16 supplies appropriate signals to some of the electrodes 14, 22 on one of the sets, and receives signals from electrodes 14 or 22 on either set. As is well-known to one skilled in the art, the manner in which the phase or amplitude of the signals is coupled across the air gap 26 provides an indication of the relative position of the substrates 12, 20 along the measurement axis. Although the outer surfaces of the electrodes 14, 22 may be exposed without any protective coating, in the past the electrodes 14, 22 of capacitive position transducers have been provided with a thin coating or covering of a dielectric material 30, 32. However, the purpose of the dielectric coating 30, 32 has been to protect the electrodes 14, 22 while continuing to allow the electrodes 14, 22 to be capacitively coupled to each other. It will be apparent from Figure 1 that there is substantial volume in the air gap 26 for both liquid and particulate contaminants to collect. Such contaminants have a dielectric constant that is significantly greater than the dielectric constant of air ( i.e., 1). Since the contaminants normally fill the gap 26 in an uneven or non-symmetrical manner, the coupling between the electrodes 14, 22 is primarily a function of the distribution pattern of the contaminants in the gap 26 rather than the longitudinal position of the substrates 12, 20 relative to each other along the measurement axis. Under these circumstances, the position transducer 10 cannot provide accurate position measurements. The problem of contaminants in the air gap 26 between the electrodes 14, 22 has been recognized. However, attempts have been made to solve this problem by reducing the size of the gap 26, thereby theoretically reducing the amount of contaminants that can be contained in the gap 26. In one conventional capacitive position transducer the air gap has been reduced to approximately 90 microns, the electrodes 22 were covered with a dielectric coating 30 having a thickness of between 30 and 70 microns and a dielectric constant of 3.8, and the electrodes 14 were covered with a dielectric coating 32 having thicknesses of between 150 and 200 microns and a dielectric constant of approximately 4. The relative impedance of the dielectric coatings 30, 32 and the air gap can be compared by comparing their ratios of thickness to dielectric constant since impedance is proportional to that ratio. The sum of the ratios of thickness of the dielectric coatings 30, 32 to their respective dielectric constants for the most favorable of this configuration is about 68 ( i.e. , [70/3.8] + [200/4]). The ratio of thickness to dielectric constant for the air gap is 90 ( i.e. , 90/1). The maximum impedance in the air gap ( i.e. , when air rather than contaminants are in the gap) is thus about 1.3 times the combined impedance of the dielectric coatings 30, 32 ( i.e . 90/68.). The ratio of the maximum gap impedance to dielectric coating impedance is even greater ( i.e ., about 2) when the thickness of the dielectric coatings 30, 32 are at the thinner ends of the ranges described above. Since the problems caused by contamination are the result of a variation in capacitive coupling that is unrelated to position, a higher percentage of variable impedance ( i.e. , gap impedance) to fixed impedance ( i.e . dielectric coating impedance) causes a greater degradation in performance for the same gap width and contaminant content. Thus, the capacitive impedance of prior art dielectric layers, being at best less than about three quarters ( i.e. , [1]/[1.30] ) of the maximum capacitive impedance in the gap has not been sufficiently high to materially reduce the problems of contamination. Attempts have also been made to minimize the contamination problem by eliminating the air gap altogether, as illustrated in the capacitive position transducer 40 shown in Figure 2. As explained in detail below, the transducer 40 of Figure 2 differs from the transducer 10 of Figure 1 by mounting the upper substrate so that the dielectric coating for the upper electrodes is in sliding contact with the dielectric coating for the lower electrodes. With reference to Figure 2, the transducer 40 includes a generally rectangular base member 42 which can be part of a caliper or machine tool, for example. The base member 42 has formed therein a rectangular cut-out receiving a support block 44. The support block 44, in turn, carries an insulating substrate 46, such as a printed circuit board. A set of electrodes 50 are mounted on the substrate 46 in a predetermined pattern of conventional design. A support frame 60 is secured to the base member 42 and encloses its upper surface along its entire length. An upper, insulating substrate 64, which may also be a printed circuit board, is mounted in the support frame 60. As will be recognized by one skilled in the art, the transducer 40 is illustrated in Figure 2 as a transverse cross-sectional view showing the structure on a plane taken perpendicularly to a measurement axis along which the lower substrate 46 moves relative to the upper substrate 64. The structure for mounting the substrate 64 in the support frame 60 is conventional and thus has been omitted for purposes of clarity. The substrate 64, like the substrate 46, carries a set of electrodes 68. As mentioned above, the transducer 40 of Figure 2 differs from the transducer 10 of Figure 1 by mounting the upper substrate 64 so that a dielectric coating 74 for the lower electrodes 50 is in sliding contact with a dielectric coating 76 for the upper electrodes 68. However, as explained above, it is necessary to mount the upper substrate 64 so that it is resiliently biased toward the lower substrate 46 but is free to move vertically. For this purpose, the substrate 64 is mounted in a substrate support 80 by flexible material 82, such as rubber, bonded between the substrate 64 and the substrate support 80. Resilient springs 84 extending between the substrate 64 and the substrate support 80 resiliently bias the substrate 64 downwardly so that the surfaces of the dielectric coatings 74, 76 are maintained in contact with each other. By eliminating the air gap altogether, the capacitive position transducer 40 of Figure 2 would appear to eliminate the problem of contaminants finding their way into an air gap between the upper and lower set of electrodes. However, because the substrate 64 is free to move vertically away from the lower substrate 46, fluid and particulate contaminants may eventually find their way between the dielectric coatings 74, 76 and displace the upper substrate 64 upwardly away from the lower substrate 46. When the upper substrate 64 is displaced upwardly away from the lower substrate 46, gaps are created between the substrates 46, 64 in the areas where the contaminant layer is not present or is thinner than the spacing between the upper substrate 64 and the lower substrate 46. For this reason, the sliding electrode concept illustrated in Figure 2 has not been entirely successful in solving the contaminant problem. One embodiment of the inventive capacitive position transducer 90 is illustrated in Figure 3. For the purposes of brevity, the components of the embodiment of Figure 3 that are identical to the conventional capacitive position transducer 10 of Figure 1 have been provided with the same reference numerals and they will not be separately explained. The inventive position transducer 90 departs from conventional wisdom by attempting to solve the contaminant problem in a manner that does not rely upon keeping the contaminants out of the air gap 26 between the electrodes 14, 22. Instead, the inventive capacitive position transducer 90 attempts to minimize the effect of the contaminants between the electrodes 14, 22. With reference to Figure 3, the electrodes 14, 22 are covered with respective, relatively thick dielectric coatings 92, 94. Although the dielectric constant of the material used for the dielectric coatings 92,94 is greater than one ( i.e. , the dielectric constant of air), the coatings 92,94 are sufficiently thick that the impedance of the dielectric coating 92,94 are greater than the impedance of the air gap 26 before it has been at least partially filled with a contaminant. The capacitive coatings 92, 94 and air gap 26 can be represented by the schematic illustrated in Figure 4, where C 1 represents the capacitance of the dielectric coating 92, C G represents the capacitance of the air gap 26, and C 2 represents the capacitance of the capacitive coating 94. In a similar manner, Z 1 represents the impedance of the dielectric coating 92, Z G represents the impedance of the air gap 26, and Z 2 represents the impedance of the capacitive coating 94. It will be apparent from Figure 4 that, if the capacitances C 1 and C 2 are much smaller than the capacitance C G , then the impedance of Z 1 and Z 2 will be much greater than the impedance of Z G . Under these circumstances, changes in the value of the capacitance C G will have relatively little effect on the impedance from terminal A to terminal B. As a result, the use of the dielectric coatings 92,94 having a high impedance ( i.e. , low ratio of dielectric constant to thickness) will make the coupling between terminals A and B relatively insensitive to variations of the capacitance C G . As mentioned above, the capacitive impedances of the dielectric coatings 92, 94 and the air gap 26 can be compared to each other by comparing their respective ratio of thickness to dielectric constant. Using this approach, the capacitive impedance of the air gap 26 will, of course vary, but the maximum capacitive impedance in an inter electrode region bounded by the capacitive coatings 92, 94 will be when air (dielectric constant = 1) is in the gap 26. When air is in the inter electrode region, the ratio of thickness to dielectric constant will be equal to the thickness of the inter electrode region since the dielectric constant of air is 1. The maximum impedance of the air gap 26 must be less than the impedance of the series combination of the dielectric coatings 92,94. The sum of the ratio of thickness to dielectric constant for both dielectric coatings 92, 94 must therefore be less than the thickness of the air gap. The maximum impedance of the air gap 26 should preferably be less than one-half of the series combination of the impedance of the dielectric coating 92,94. The sum of the ratio of thickness to dielectric constant for both dielectric coatings 92, 94 is, therefore, preferably more than twice the thickness of the air gap. Finally, it is desirable for the maximum impedance of the air gap 26 to be less than about one-third of the series combination of the impedance of the dielectric coating 92, 94. It is therefore desirable for the sum of the ratio of thickness to dielectric constant for both dielectric coatings 92, 94 to be more than three times the thickness of the air gap. Thus, if the dielectric constant of both dielectric coatings 92, 94 is 4, the sum of the thicknesses of the dielectric coatings 92, 94 must be at least four times the thickness of the air gap 26, preferably 8 times the thickness of the air gap 26, and most desirably 12 times the thickness of the air gap 26. In the example given, if contaminants in the gap 26 cause the impedance of the air gap to fall from 1,000 ohms to 500 ohms, the overall impedance across the gap will change from 2,000 ohms to 1,500 ohms. Thus, even if the contaminants cause the impedance across the air gap 26 to decline by 50%, the overall impedance between the electrodes 14, 22 will change by only 25%. However, in the preferred situation where the combined impedance of the capacitive coatings 92, 94 is twice the maximum impedance of the air gap, the contaminants would cause the impedance between the electrodes 14, 22 to change from 3,000 ohms to 2500 ohms. Thus, in the preferred situation, a change in gap impedance of 50% would result in a change in impedance between the electrodes 14, 22 of only 16.7%. Finally, in the most desirable configuration where the combined impedance of the capacitive coatings 92, 94 is three times the maximum impedance of the air gap, the contaminants would cause the impedance between the electrodes 14, 22 to change from 4,000 ohms to 3,500 ohms. Thus, in the most desirable situation, a change in gap impedance of 50% would result in a change in impedance between the electrodes 14, 22 of only 12.5%. While dielectric coatings for the electrodes 14, 22 of capacitive position transducers have been used, as explained above, there was no appreciation that the dielectric coatings, if made sufficiently thick, could reduce the effect of contaminants between the electrodes 14, 22. As a result, the capacitive coatings were used only to protect the electrodes, and they were thus too thin relative to the practical air gap to have any material effect on the problem of contaminants in the air gap. An alternative embodiment of an inventive capacitive position transducer 100 is illustrated in Figure 5. The position transducer 100, like the position transducer 90 of Figure 3, utilizes relatively thick dielectric coatings 92, 94. However, the capacitive position transducer 100 of Figure 5 utilizes a sliding contact between the dielectric coatings 92, 94. For this purpose, the position transducer 100 utilizes the same substrate, electrode and suspension structure that is used in the previously described position transducer of Figure 2. Specifically, the upper substrate 64 is mounted in a substrate support 80 by flexible material 82, and it is resiliently biased in a downward direction by springs 84. In the embodiment 100 of Figure 5, the series impedance of the capacitive coatings 92,94 should be at least equal to the maximum impedance between the coatings 92, 94 when the practical limits of surface roughness and flatness have forced the coatings 92, 94 apart from each other to create air gaps in the inter electrode region bounded by the capacitive coatings 92, 94. Therefore, the sum of the ratio of thickness to dielectric constant for both dielectric coatings 92, 94 must be less than the maximum ratio of thickness to dielectric constant in the inter electrode region bounded by the coatings 92, 94. As explained above, the maximum ratio of thickness to dielectric constant in the inter electrode region will occur when air is in the inter electrode region. Since the dielectric constant of air is 1, the maximum ratio of thickness to dielectric constant in the inter electrode region is simply the thickness of the inter electrode region bounded by the dielectric coatings 92, 94. The inter electrode region will always have some thickness as a result of either practical manufacturing constraints or the dielectric coatings 92, 94 being forced apart from each other by contaminants . Thus, the sum of the ratio of thickness to dielectric constant for both dielectric coatings 92, 94 must be less than the thickness of the inter electrode region bounded by the coatings 92, 94. Although the series impedance of the dielectric coatings 92, 94 must be at least equal to the maximum impedance in the inter electrode region between the coatings 92, 94, the series impedance of the dielectric coatings is preferably at least twice the maximum impedance between in the inter electrode region between the coatings 92, 94 as manufactured and also when fluid contaminants have entered between the coatings 92, 94. Finally, the series impedance of the dielectric coatings 92, 94 is most desirably at least three times the maximum impedance between the coatings 92, 94 as manufactured and also when contaminants have entered between the coatings 92, 94. As mentioned above, practical capacitive transducers using the "sliding contact" approach still create air gaps in the range of 0.002 - 0.004 mm. The thinnest dielectric coating 92, 94 could be used when an air gap of 0.002 mm was achieved. Larger air gaps would require thicker dielectric coatings 92, 94. Of course, larger air gaps caused by contaminants increasing the spacing between the dielectric coatings 92, 94 would require significantly thicker dielectric coatings 92, 94. Using an example of dielectric coatings 92, 94 having a dielectric constant of 4, the sum of the thickness of the dielectric coatings 92, 94 must be at least 0.008 mm, is preferably at least 0.016 mm, and is most desirably at least 0.024 mm. It is thus seen that the inventive capacitive position transducer seeks to eliminate the effects of contaminants rather than the contaminants themselves. The inventive transducer is able to accomplish this function for a wide variety of contaminant conditions merely by adjusting the thickness or dielectric constant of the coating. Moreover, the technique of using a thick dielectric coating for the electrodes of capacitive position transducers can be used to solve this type of contamination problem for virtually any type of capacitive position transducer. It will therefore be understood that the capacitive position transducers and specific coating thicknesses described herein are for illustration only and should not be used limit the scope of the claims.

Claims: A capacitive position transducer having a first substrate (12,80) containing a first set of electrodes (14,68) arranged along a measurement axis, and a second substrate (20,46) containing a second set of electrodes (22,50) arranged along the measurement axis, the first and second substrates being positioned with the first and second sets of electrodes opposite each other so that the capacitive coupling between the first and second sets of electrodes provides an indication of the relative position between the first and second substrates along the measurement axis, wherein a layer of dielectric material (92,94) coats at least one set of first and second sets of electrodes thereby forming an air gap (26) between an uncoated set of electrodes and the dielectric material when only one set of electrodes is coated, or between respective dielectric materials when both sets of electrodes are coated; characterised in that the ratio of thickness to dielectric constant of the dielectric when only one set of electrodes is coated, or the sum of the ratios of thickness to dielectric constant for each dielectric material when both sets of electrodes are coated is greater than the thickness of the air gap (26), so that the maximum capacitive impedance between the first and second sets of electrodes in the air gap is less than the capacitive impedance between the first and second sets of electrodes in the dielectric material. A transducer according to claim 1, wherein both the first and second sets of electrodes (14,68;22,50) are coated with the dielectric material (92,94). A transducer according to claim 2, further including mounting means (42,60) positioning the first and second substrates (44,80) so that the dielectric materials (92,94) coating the first and second set of electrodes (68,50) are in sliding contact with each other as the first and second substrates move relative to each other along the measurement axis. A transducer according to claim 3, wherein the mounting means comprises a frame member (60) movable with respect to the second substrate (46) along the measurement axis; a resilient support (82) mounting the first substrate (80) in the frame member so that the first substrate can move towards and away from the second substrate; and a spring member (84) resiliently biasing the first substrate towards the second substrate thereby causing the dielectric material (92) coating the first set of electrodes (68) to slide along the dielectric material (94) coating the second set of electrodes (50) while allowing the first and second sets of electrodes to move towards and away from each other as the first and second sets of electrodes move relatively to each other along the measurement axis. A transducer according to claim 3 or claim 4, wherein the air gap (26) is less than or equal to 0.004 mm. A transducer according to claim 5, wherein the sum of the ratios for each dielectric material (92,94) between the first and second set of electrodes (68,50) is greater than 0.008 mm. A transducer according to claim 5, wherein the sum of the ratios for each dielectric material (92,94) between the first and second set of electrodes (68,50) is greater than 0.012 mm. A transducer according to any one of the preceding claims, wherein the sum of the ratios for the or each dielectric material (92,94) between the first and second set of electrodes (14,68,22,50) is at least twice the maximum ratio of thickness to dielectric constant in the air gap (26) so that the maximum impedance between the first and second sets of electrodes in the air gap (26) is less than one-half of the impedance between the first and second sets of electrodes in the dielectric material. A transducer according to any one of the preceding claims, wherein the sum of the ratios for the or each dielectric material (92,94) between the first and second set of electrodes (14,68,22,50) is at least three times the maximum ratio of thickness to dielectric constant in the air gap (26) so that the maximum impedance between the first and second sets of electrodes in the air gap (26) is less than one-third of the impedance between the first and second sets of electrodes in the dielectric material.

Inventors: ANDERMO NILS I, ANDERMO, NILS I.
Applicants: MITUTOYO CORP, MITUTOYO CORPORATION
Application year: 1994
Main classification: G01D5
IPC codes: G01B7, G01D5, G01B3